According to the way in which the stone naturally lends itself, we have various types of rubble walls. The commonest is the rough rubble wall in which the stones have neither regular shapes nor regular sizes, or even courses. The wall is composed of large stones and small stones (the latter are called spalls, and fill in the interstices between the larger stones). The joints of mortar between the stones may be plastered roughly over the surface, covering much of the face of the stones themselves, or they may be roughly but neatly pointed with white mortar, or the joints may be raked out. Where the stone has a natural tendency to cleave into long, flat shapes, the rough rubble may become more regularly coursed in appearance. All of these types are respectively illustrated in Figures 1, 2, 3, and 4.
A softer stone, which can be dressed with the hammer, may be treated in two different ways: It may be shaped to fit closely, without using any spalls to fill up the interstices, and, thus, appear as a cut-out puzzle; this is called "cobweb rubble." However, the more dignified treatment is the squared, uncoursed rubble, in which the blocks are cut to rectangular shape and the joints pointed with a tool. Figures 5 and 6 illustrate these.
A wall built entirely of field stone depends upon the mortar for its strength. It appears the best when the joints of the surface are raked out, permitting a large part of the stones to project outward. Figure 7 illustrates this kind of rubble wall.
When the rubble wall is built with very carefully squared stones, and in regular courses, it partakes more of the monumental character of ashlar work and draws away from the rustic value of rubble. In determining the amount of cutting which is to be done, the character of the building should be considered, remembering that the smoother and more finished the wall, the more monumental is its appearance.